1. Field of the Invention
This invention relates to seismic detection apparatus and to a method of seismic exploration while drilling.
2. Description of the Related Art
In conventional seismic detection, a seismic source signal is reflected from different rock substrates or strata within the earth, so producing compressional waves and shear waves which can be analyzed to determine the direction and extent of geological features in strata.
It is possible to use a drill bit to produce a seismic source signal during drilling and techniques of analyzing these signals to produce information on rock formations have been investigated. The basic principle of seismic while drilling technology includes a working drill bit that destroys the rock at the bottom of a borehole, thereby radiating acoustic energy into surrounding geological formations. Some of this energy travels directly to the surface where it can be detected by geophones, or by hydrophones if the well is offshore. Some of the energy radiates downwards ahead of the bit where it may be reflected by impedance contrasts in the earth. This reflected energy is also detected at the surface. However in certain drilling conditions, for example when drilling horizontal wells, using standard seismic while drilling techniques is difficult as acoustic reflections from the substrates are not reflected towards the surface. A look-ahead method within the context of a measurement while drilling tool is described for example in U.S. Pat. No. 5,678,643. A VSP based look-ahead method within the context of a measurement while drilling tool is described for example in EP-A-0795764.
The present invention aims to provide apparatus and an associated processing method that enable collection and analysis of borehole seismic measurements while drilling, in particular, for coiled tubing systems and in the drilling of horizontal wells.